Acoustic method and apparatus for drilling boreholes



April 25, .1967 w, BROOKS I 3,315,755

ACOUSTIC METHOD AND APPARATUS FOR DRILLING BORE-HOLES Filed June 7, 1965 2 Sheets-Sheet 1 FLOW CONTROLLER SPEED CONTROLLER ACOUSTIC METHOD AND APPARATUS FOR DRILLING BOREHOLES Filed June 7, 1965 A ril 25, 1967 w.. B. BROOKS 2 Sheets-Sheet 2 PREs uRE SENSOR FIG. 4.

United States Patent Q 3,315,755 ACOUSTIC METHOD AND APPARATUS FOR DRILLING BOREHOLES Warren B. Brooks, Dallas, Tex., assignor to Mobil Oil Corporation, a corporation of New York Filed June 7, 1965, Ser. No. 470,265 14 Claims. (Cl. 17556) This application is a continuation-in-part of my copending application Ser. No. 203,076, filed June 18, 1962, entitled, Acoustic Method and Apparatus for Drilling Boreholes, and now abandoned.

This invention relates to drilling boreholes in an earth formation. More specifically, this invention relates to drilling boreholes wherein the borehole is advanced through the formation by subjecting the formation to alternating stresses due to particular pressure conditions set up in a drilling fluid at the bottom of the borehole.

The most widely accepted and presently used method of drilling boreholes, particularly in the oil industry, is that referred to as rotary drilling. In rotary drilling, a drill bit is secured to the lower end of one or more sections of pipe called drill collar, which in turn is secured to a string of pipe referred to as the drill stem. While drilling fluid is pumped through the drill stem and drill collar into contact with the formation through the drill bit, the drill bit is rotated by the drill stem from surfacernounted equipment. The earth material comprising the formation is subjected to alternating stresses by tooth cutters on the drill bit which effect chipping, fracturing, and abrading of the material. The conventional drill bit depends to a great extent for its cutting action upon the compressive stresses induced by excessive weight placed on the bit by the weight of the drill collar. Numerous difliculties are inherent in rotary drilling methods. The necessity for weight on the bit results in fatigue failures of the drill collars, along with tooth and bearing wear of the bit itself. Appreciable power is lost in transmitting torsion from the surface through the drill stem and drill collars to the bit at the bottom of the hole. Also, time is lost in going in and coming out of the hole at times when it is necessary to make changes in the bit. Many of these problems may be minimized, if not entirely eliminated, by a method of drilling in which a drill bit is not employed and there is no necessity for rotation of long strings of drill pipe and drill collars.

It is one object of the present invention to provide an improved method of drilling a borehole in an earth formation. It is another object of the present invention to provide a method of drilling wherein a drill bit is not employed. It is a further object of the invention to provide a drilling method in which the cutting of the formation is accomplished by means of alternating acoustic pressure pulses generated within a body of drilling fluid adjacent the bottom of the borehole. It is another object to provide a borehole drilling method employing subcavitational acoustic pressure pulses. It is another object of the invention to provide a method of drilling a borehole by acoustic pressure pulses in a drilling fluid within the cavitational range. It is a further object of the invention to provide apparatus for carrying out the method of the invention. These and still further objects of the invention will be evident from a reading of the following description taken in conjunction with the accompanying drawings.

In accordance with one aspect of the invention, drilling fluid is conducted downwardly through a borehole, and at the bottom of the borehole, alternating acoustic pressure pulses are generated within the drilling fluid. The acoustic pressure pulses generated Within the drilling fluid may be effected under cavitational or subcavitational ice conditions. Preferably the acoustic pressure pulses are generated in the range of cavitation, i.e., at or closely approaching cavitational conditions. The acoustic pressure pulses generated in the drilling fluid effect failure of the formation, and the resulting cuttings are washed away and carried to the surface by the drilling fluid. Since high flow rates for the drilling fluid cause turbulence and make it diificult to operate in the range of cavitation, it is preferred that the flow rate of the drilling fluid beneath the means for generating the acoustic pressure pulses be approximately the minimum rate necessary to cause the cuttings to flow up around said means into a zone from which the main fluid circulation may carry the cuttings out of the borehole.

In accordance with another aspect of the invention, a drilling tool is provided which comprises in combination conduit means for flowing drilling fluid in a confined flow path through a borehole, reciprocable means secured to the lower end of the conduit means for generating alternating acoustic pressure pulses in the drilling fluid, and means connected to the reciprocable means for actuating it at a desired frequency.

In the drawings:

FIGURE 1 is a view partially in section of fluid turbine-powered apparatus for carrying out the invention.

FIGURE 2 is a segmental view, partially in section, of a modified version of the apparatus illustrated in FIGURE 1.

FIGURE 3 shows in partial section and perspective a magnetostrictive-actuated apparatus which may be employed in carrying out the method of the invention.

FIGURE 4 illustrates in section a jet-edge generator actuated form of apparatus which may be used in the method of the invention.

FIGURE 5 is a view in cross section taken along the line 5-5 of FIGURE 4.

FIGURE 6 is a view in cross section taken along the line 66 of FIGURE 4.

FIGURE 7 is a perspective view of a portion of one element of the apparatus shown in FIGURE 4.

FIGURE 8 is a diagrammatic representation of a vibration dampener in a drill stem used in the invention.

FIGURE 9 is a view in section of a form of vibration dampener which may be used.

Each apparatus illustrated in the various figures of the drawings may be employed in carrying out the method of the invention. For purpose of simplicity of description, those portions of apparatus which are common to the various embodiments illustrated will be given identical reference numerals.

Referring specifically to FIGURE 1, a fluid turbinepowered acoustic pressure pulse generator is shown in drilling position within a borehole 10. Outer tool casing or housing 11 is secured to the lower end of drill string 12. Positioned within casing 11 supported on a bearing 13 is a multielement fluid turbine rotor 14 to which is connected a shaft 15. Secured on the lower end of shaft 15 is a bevel gear 20 which meshes with a similar bevel gear 21 which is fixed to a crankshaft 22. Crankshaft 22 is mounted on bearings 23. Secured to crankshaft 22 is a connecting rod 24, the lower end of which is afiixed to slide assembly 25. The gears, crankshaft, connecting rod, and associated parts are all housed within a casing or gearbox 30 which is supported within casing 11 by a plurality of brackets 31. Brackets 31 are so positioned around the gearbox that ample space is left within casing 11 to permit the flow of drilling fluid from the turbine rotor downwardly to the lower portion of the tool. Secured to the lower end of slide assembly 25 is a piston rod 32. A fluid-tight seal 33 is positioned around the piston rod at the lower end of the gearbox 30.

A fluid turbine of the type illustrated herein is shown in greater detail in US. Patent No. 2,554,005 to A. G. Bodine, Jr. Any fluid turbine which will generate the necessary power may be employed in the application shown in FIGURE 1. It has been found that a turbine having six stages as illustrated would generate up to about 200 horsepower, which is adequate for carrying out the present invention.

.Connected to the lower end of piston rod 32 is a piston 34 which extends across the lower, open end of casing 11. Piston 34 is provided with a plurality of openings or passageways 35 which function to permit flow of drillingfluid from within casing 11, through the lower end of the tool, and direct it against the formation being drilled. Secured around the top side of piston 34 and fitting within the lower end of casing 11 is a skirt 40 which fits in sliding relationship with the lower inside surface of casing 11. If it is desired that a portion of the drilling fluid be allowed to by-pass the piston, skirt 40 may be provided with one or more openings 41.

While the provision of openings 35 is presently preferred; it is also possible to omit these openings in the piston. In this case, the drilling fluid may pass downwardly in skirt 40, radially outwardly over the top surface of the piston 34, beneath piston 34, to pick up cuttings, and may then pass upwardly in the borehole to remove the cuttings.

In another embodiment, to minimize the possibility of drilling fluid bypassing the bottom of the borehole beneath such a solid piston; the piston may be of slightly smaller diameter than that of skirt 40 in order to provide a flow path directly to the bottom of the borehole. Alternatively, the solid piston may be positioned eccentrically with respect to skirt 40 to provide a flow path to the bottom of the borehole. In this case, means may be provided to move the piston in a closed arcuate path during its vertical reciprocation so that the acoustic pressure pulses will uniformly act on the borehole bottom.

In lieu of a homogenous drilling liquid, it is also within the scope of the invention to employ a high density non-circulating drilling liquid in the bottom of the borehole which will float the cuttings up to a zone from which the cuttings may be removed by a low density drilling fluid from within skirt 40 as described above.

Rotation of turbine rotor 14 is caused by the flow of drilling fluid from drill string 12 into casing 11. A suitable flow controller, which is schematically illustrated in FIGURE 1, may be employed to control the rate of fluid flow to thereby regulate the rotation of turbine rotor 14 and to in turn regulate the reciprocation of piston 34. The rotation of the turbine is translated into longitudinal motion to effect reciprocation of the piston 34 by means of gears 20 and 21 which cause the turning of crankshaft 22, resulting in the reciprocation of connecting rod 24. The connection of connecting rod 24 to piston rod 32 through slide assembly 25 effects the translation of the motion into the longitudinal reciprocating action of rod 32. The drilling fluid after passing through the turbine rotor 14 continues flowing downwardly within outer casing 11 and passes outwardly against the formation through openings 35 in piston 34. Reciprocating action of the piston generates acoustic pressure pulses within the drilling fluid, which pressure pulses are superimposed on the hydrostatic pressure of the drilling fluid. The piston may be reciprocated under conditions which will effect a state of cavitation within the drilling fluid, for example, by controlling the flow of the drilling fluid. Reference numeral 42 denotes the bubbles or cavities which may be induced in the drilling fluid, if so desired, by reciprocation of the piston.

As indicated above, the means for generating acoustic pressure pulses in accordance with'the present invention are spaced closely adjacent the bottom of the borehole. For example, it is preferred that piston 34 at the bottom of its stroke be spaced between about 2 millimeters to 2 borehole diameters from the bottom of the borehole.

The apparatus illustrated in FIGURE 2 is identical to that shown in FIGURE 1 with the exception that an electric motor is substituted for the fluid turbine. Referring specifically to FIGURE 2, electric motor 50 is supported within outer casing 11 on brackets 51 which may be two or more in number and are so positioned and of such size that the drilling fluid may readily flow around the motor and brackets to the lower portion of casing 11. All other elements of the apparatus illustrated in FIGURE 2 are identical to those shown in the apparatus of FIGURE 1. Electric motor 50 is connected to shaft 15 and serves to rotate shaft 15 to effect reciprocation of piston 34 in the manner and for the purposes above described in connection with FIGURE 1. A suitable speed controller as schematically illustrated in FIGURE 2 may be operatively connected to the motor 50 to thereby regulate the reciprocation of piston 34 as desired, for example, to effect cavitation or vary the velocity of the piston in correlation with the depth of the borehole.

Shown in FIGURE 3 is apparatus in which a magnetostrictive element is employed as the prime mover for effecting reciprocation of piston 34. In FIGURE 3, for purposes of simplicity, piston skirt 40, positioned on the upper surface of piston 34, has not been illustrated, though it is to be understood that the piston skirt may be employed in this embodiment of the apparatus in a form identical to that shown in FIGURE 1. Referring specifically to FIGURE 3, a magnetostrictive device 60 is positioned within and secured to casing 11 by means of brackets 61 which will allow the flow of drilling fluid past the magnetostrictive device to the lower end of the outer casing 11. Electric current is supplied to the magnetostrictive device by means of cable 62. The lower end of the magnetostrictive device is secured to the upper face of piston 34. Device 60 may be any of the known magnetostrictive units, such as one described in U.S. Patent No. 2,858,108 to B. A. Wise. The particular magnetostrictive device illustrated comprises a plurality of magnetostrictive elements 63, which may be formed of a nickel alloy wrapped with wire forming coil 64 through which current flows from the cable 62. The magnetostrictive elements are enclosed by an outer casing 65 which is sealed at its upper end around cable 62 and secured at its lower end to the piston 34. Casing 65 may be filled with oil to provide an oil bath around the magnetostrictive elements and the coil to aid in dissipating the heat generated by operation of the device. Flow of electrical current supplied from a generator positioned on the surface of the earth through coil 64 causes alternate expansion and contraction of the magnetostrictive elements 63, resulting in reciprocating action of piston 34. While the magnetostrictive expansion or contraction of the elements is of itself extremely rapid and results in high frequencies, the cycles at which it is pulsed can be tuned to the resonant frequency of the magnetostrictive section. The resonant frequency of the section is determined by its length, density, and modulus of elasticity. For most effective operation, the frequency at which the alternating current is supplied through the power cable 62 will be one-half the resonant frequency of the assembly of magnetostrictive elements. That is, each time the current reaches a maximum and returns to zero, there will be a complete cycle of expansion and contraction of the magnetostrictive device. Similarly, on the negative cycle of the alternating current where the current reaches a minimum and returns to zero, there will be a complete cycle of expansion and contraction of the magnetostrictive device. A direct current may be applied to the coil simultaneously with the alternating current to attain most eflicient magnetostrictive action. By utilizing a combination of alternating and direct current to actuate the magnetostrictive device, greater efficiency of the magnetostrictive section is realized. In this application, the magnetostrictive section must be designed such that one complete cycle of the current is synchronized with one complete cycle of the mechanical oscillation of the section.

The electrical power necessary to effectively operate such a magnetostrictive device in carrying out the method of the invention will be something on the order of magnitude of approximately 200 kilowatts. Just as in the operation of the apparatus of FIGURES 1 and 2, the drilling fluid from the drill string flows downwardly through casing 11, around the magnetostrictive device 60, and outwardly through openings 35 in piston 34 into contact with the formation being drilled. The piston 34 is actuated by the magnetostrictive device to effect acoustic pressure pulses in the drilling fluid prior to its contact with the formation.

Another form of apparatus which may be utilized in carrying out the method of the invention is illustrated in FIGURES 4-7. This particular form of apparatus is sometimes referred to as a jet-edge generator because of the vibratory action obtained by streams of drilling fluid flowing through the device and striking the knifelike edges of portions of the apparatus of the device.

Referring specifically to FIGURE 4, outer casing 70 is secured to the lower end of drill string 12. Secured within housing 70 forming a closure completely across the interior of the housing is member 71 which is provided with openings 72 and conduits 73. The positions of the openings 72 and conduits 73 in member 71 may best be seen by reference to FIGURE 5. Portions 74 of member 71 extend downwardly from the main portion of the memher to provide the necessary structure forming the conduits 73, which at their lower ends turn inwardly and form nozzlelike outlets. Appended downwardly from member 71 are two support members 75 to which is secured vibrating bar 80. Each end of vibrating bar 80 is provided with a knifelike edge. Piston 34 is secured to vibrating bar 80 by connecting member 81. Piston 34 may be provided with an upwardly extending skirt 82 which fits in sliding relation with the outer surface of housing 70 and slightly overlaps the lower end of the housing. For purposes of reference in describing the design of the vibrating bar, the points of connection of members 75 to the vibrating bar shall be referred to as the node points, and the point of connection of member 81 to the vibrating bar shall be referred to as the antinode point. The node points are those points about which the bar vibrates and consequently the points at which there is no deflection of the bar during vibration, whereas the anti-node point is that point along the center line of the bar which experiences the greatest vibration or the greatest amount of amplitude.

In operating the jet-edge generator illustrated in FIG- URES 47, the drilling fluid flows downwardly through the drill string into the housing 70, continuing its flow through both openings 72 and conduits 73 in member 71. That fluid passing through openings 72 simply continues downwardly through passages 35 in piston 34 and into contact with the formation being drilled. The portion of the drilling fluid flowing through passages 73 flows out the nozzlelike openings at the lower end of passages 73 and directly into contact with the knifelike edges of the vibrating bar 80. Contact of the drilling fluid at high velocity with the knifelike edges of the vibrating bar causes the bar to vibrate, and, since piston 34 is secured to the point of maximum deflection of the vibrating bar, the piston itself is then caused to vibrate or reciprocate at the frequency of oscillations being experienced by the vibrating bar. After contact with the knifelike edges of the vibrating bar, the drilling fluid from passages 73 continues its path downwardly through piston 34 in openings 35, into cont-act with the formation. The reciprocating action of the piston 34 generates alternating acoustic pressure pulses in the body of drilling fluid below the piston flowing into contact with the formation being drilled.

For optimum operation of the jet-edge generator, the

apparatus preferably is designed in the following manner, with the design designations being given with reference to FIGURE 7. The angle a at which the knifelike edges of the ends of vibrating bar are out should be approximately 30. The distance 0 between the node points, that is, the distance between the center lines of the support members 75, should be 0.49 times the total length e of the vibrating bar 80 (c=49e). The total length of the vibrating b-ar, e, should be at least twice the width, b, of the vibrating bar (e22b). The width a of the nozzlelike openings of passages 73 should be no greater than the thickness 1 of the vibrating bar 80 (agf). The frequency of the vibrations obtainable with the vibrating bar, the dimensions of the vibrating bar, the velocity of the drilling fluid flowing through the nozzles from passages 73, and the distance of the nozzles from the knife edges of the vibrating bar are all related in accordance with the following formula:

where f=the frequency of vibration in cycles per second,

t=the thickness of the vibrating bar in feet,

e=the length of the vibrating bar in feet,

C =the approximate velocity of sound in the material of which the vibrating bar is constructed,

U=the velocity of fluid flow through the nozzles in feet per second, and

d=the distance of the nozzles from the knife edges of the the vibrating bar in feet.

As a practical matter, most holes which may be drilled with the apparatus will be of such size that the vibrating bar 80 may have a length ranging between 4 and 12 inches. For purposes of example, assuming a vibrating bar 6 inches or 0.5 foot long, having a thickness of /2 inch or 0.0416 foot, and constructed of a material in which the velocity of sound is 17,000 feet per second, the frequency of vibration of the bar is determined as follows:

Obviously, in accordance with the above formulas, the velocity of fluid flow from the jets, U, and the distance of the jets from the knife edge of the vibrating bar, d, may be established as desired to correlate with the vibrating frequency of the bar. Also, it will be readily recognized that the vibrating frequency of the bar may be established at the desired rate by variation in its different dimensions. The frequency at which the bar should preferably vibrate will be understood from discussions hereinafter.

It will be apparent that with any vibrating system, such as those disclosed herein, there will be transmission of the vibrations up to the drill stem, which not only results in a wasteful dissipation of power but tends to cause failure of the drill stem joints. -It is, therefore, preferred that means for reflection of the vibrations be positioned in the drill stem above the location of the vibration generator. In its simplest form, a reflection means as illustrated in FIGURE 8 is positioned in the drill string. Illustrated in FIGURE 9 is a preferred form of the vibration reflector which comprises a neoprene sleeve 91 in which is encased a rather heavy spring 92. The spring should be of suflicient strength to withstand the torque and vibratory forces to which the drill string is subjected.

Any one of the above-described tools may be utilized to generate acoustic pressure pulses in the drilling fluid in accordance with the invention. The tools may be operated under such conditions that the acoustic pressure pulses will be either of the cavitation-producing type or the noncavitation-producing type. Obviously, other forms of vibration-producing apparatus, such as are often referred to as acoustic transducers, may be used in lieu of those described herein.

As previously indicated, the method of the invention may be carried out under cavitating conditions. In such a case, the apparatus utilized for generating acoustic pressure pulses is operated in such a manner that a state of cavitation is established in the drilling fluid or, in other words, a plurality of bubbles or cavities are formed in the drilling fluid beneath the piston 34. The cavities occur due to severe reduction of the pressure within the fluid. The pressure within each of the cavities will be approximately that of the vapor pressure of the fluid in which the cavity is formed. While this pressure would generally be very low, it will still be at a slight positive pressure. For calculation purposes, however, it may be assumed that the cavities will form at least by the time the pressure closely approaches zero. When the cavities so formed are subjected to hydrostatic pressure or hydrostatic pressure coupled with a positive acoustic pressure, the cavities collapse. When the positive pressure creating the collapse is high, localized pressure of thousands of pounds per square inch and localized temperatures of thousands of degrees can be created in the cavitating region. The higher temperatures and pressures inherent in the collapse of the cavities effects fracturing of the rock of the formation with the result that a borehole may be drilled without the necessity of using a drill bit.

The various previously described apparatus may be operated under conditions which will produce a state of cavitation in the drilling fluid in accordance with the following formula:

where V=the maximum velocity of the piston 34, P =the static pressure within the drilling fluid, g=the acceleration produced by gravity, =the density of the drilling fluid, and C=the speed of sound in the drilling fluid.

Assuming that the apparatus is operating in a drilling fluid weighing about 12 pounds per gallon under a static pressure of about 4500 p.s.i. (probably at about 6500 feet in depth) and with the speed of sound in the drilling fluid being about 5000 feet per second, a condition of cavitation is established in the drilling fluid when (4500 lbs/in?) (32.2 fix/sec?) ft 2 12 lbs./gal.=93.6 lbs/ft?) (5000 ft./scc.)

44.4 ft./see.

Assuming the Waveform of the alternating pulses is sinusoidal, the piston displacement is expressed in the formula where V=velocity amplitude in feet per second, f=frequency in cycles per second, and X=maximum amplitude of the vibrating piston in feet.

At 100 cycles per second, the minimum distance the piston must move to induce cavitation is 0.07 ft.z0.85 inch 8 duce a state of cavitation in the drilling fluid, that the velocity of piston 34 be increased. This is true because the velocity of the piston in order to induce cavitation is directly proportional to the static pressure at the point where the cavitation is being induced in the drilling fluid.

Where the apparatus is to be employed under cavitating conditions, it will be necessary that those surfaces of the apparatus such as the lower face of piston 34 be suffi ciently resistant to the cavitaton that they will not be eroded by any bubbles which may collapse in the vicinity of the apparatus itself. For example, a rock interface may withstand impact pressures from 1200 to 5000 p.s.i., while the face of the piston must be of such material that it can withstand much higher pressure. The face of the piston preferably is, therefore, of a corrosion-resistant material having high tensile and compressive strength and which has been annealed in order to relieve the stresses in the surface material. Stainless steels and such exotic metals as titanium will perform this function very satisfactorily.

It should also be noted that cavitation is much easier to obtain where there are nuclei in the liquid around which the bubbles or cavities can form. In this respect, conventional drilling mud having fine-grain weighting agents affords a very fine media in which to create cavitation. It should also be pointed out that the force of the impact energy imparted by the collapsing bubbles is increased as the surface tension of the fluid is increased in which they are created and collapse. For this reason, water or aqueous solutions are particularly well suited to the production of violent cavitation because of the very high surface tension of such fluids.

In the event it is not desired that a state of cavitation be induced in the drilling fluid below the piston 34, the piston may be operated at a velocity below that indicated above required for inducing a state of cavitation. Operating the piston at such a subcavitational level will induce alternating acoustic pressure pulses within the flowing drilling fluid below the piston, though the pressure pulses will not be so violent as to set up a condition of cavitation. Such alternating acoustic pressure pulses superimposed on the hydrostatic pressure of the flowing fluid will effectively fracture the formation and thus drill a borehole, though not at such a rapid rate as might be obtained under conditions of cavitation.

Preparatory to drilling a borehole with the apparatus and method of the present invention, it may be preferred that the borehole be initially drilled by conventional means to a depth of several hundred feet. The drill string is then withdrawn from the borehole and the bit is replaced with a drilling tool as previously described. The drill string and drilling tool then are lowered into the borehole until piston 34 of the drilling tool is positioned in the vicinity of the bottom of the borehole. The drill ing fluid flows downwardly through the drill string and drilling tool, and returns carrying the cuttings to the surface in the annular space around the drilling tool and drill stem within the borehole. Where the drilling tool is of the fluid turbine or jet-edge generator actuated type, the piston 34 will be reciprocated when the drilling fluid has reached sufficient velocity through the tool. In the case of the electric motor or magnetostrictive device actuated type drilling tool, the reciprocation of piston 34 is effected independent of the flow of drilling fluid. In each instance, however, actuation of the piston 34 generates alternating acoustic pressure pulses in the drilling fluid, such pressure pulses being superimposed on the hydrostatic pressure of the drilling fluid in order to effect fracturing of the rock or other material comprising the formation being drilled. The cuttings severed from the formation are carried by the drilling fluid upwardly to the surface through the annular space around the drilling tool and drill string. As drilling progresses, the drilling tool is lowered in accordance with the rate of drilling in order to maintain it in suflicient proximity to the lower end or bottom of the borehole to most effectively carry out the drilling operation.

In the event that it is desired that the drilling operation be carried out under cavitating conditions, the piston 34 of the drilling tool is operated at a velocity sufficient to generate cavities within the drilling fluid below the lower surface of the piston, such cavities flowing with the drilling fluid which is being directed against the formation being drilled. Each series of cavities is actually generated within the drilling fluid on the upstroke or the stroke away from the bottom of the borehole of the piston 34. Due to the rapid alternate strokes made by the piston, the cavities will tend to oscillate within the drilling fluid as they flow downwardly into contact with the formation. During this oscillation, the cavities will tend to enlarge, and as they approach or contact the formation the hydrostatic pressure within the drilling fluid will tend to collapse the cavities. Obviously, for each upstroke of the piston 34 there must be a downstroke which generates a positive acoustic pressure pulse Within the drilling fluid. This positive acoustic pressure pulse travels downwardly in the drilling fluid at approximately the rate of the speed of sound within the drilling fluid. The positive acoustic pressure pulse in cooperation with the hydrostatic pressure within the drilling fluid effects complete and sudden collapse of the cavities with the resultant high temperatures and pressures which effect fracture of the formation. It is preferred that the lower face of piston 34 be maintained at a distance from the formation which will permit the hydrostatic pressure and the positive acoustic pressure pulses generated by the piston to collapse the cavities substantially at the surface of the formation, or preferably actually within the pores of the formation to most effectively fracture the formation. The drilling tool and drill string are constantly lowered at a rate commensurate with the rate of fracturing of the formation in order to maintain the lower face of piston 34 at a substantially constant distance from the bottom of the borehole as drilling progresses. The cuttings removed are carried with the drilling fluid back to the surface in the manner previously described. As drilling progresses to greater depth, it will be obvious from the previous discussions of the operation of the drilling tools employed that it may be necessary to increase the velocity of the piston 34 in order to continue to maintain a state of cavitation in the drilling fluid. In the drilling fluid actuated types of tools, this, of course, may be done by increasing the flow of drilling fluid to the tool, for example, by the flow controller of FIGURE 1. When using the electrically powered tools, it will of course be necessary that the conditions of operations of the tools be changed in order to increase the velocity of the piston to the rate necessary at the depth being drilled, for example, by the speed controller of FIGURE 2. Suitable means may be employed to sense the spacing between the piston and the bottom of the borehole, for example, distance measuring means, a pressure sensor connected to the bottom of the drill string as shown schematically in FIG- URE 4, or downwardly extending fingers secured to the bottom periphery of skirt 40 which may also function as reamers.

While preferred embodiments of the present invention have been shown and described, it will be appreciated that the invention is not limited to the specific embodiments described. Accordingly, it is intended to encompass all changes and modifications as fall within the scope and spirit of the appended claims.

What is claimed is:

1. In apparatus for drilling a borehole in an earth formation, the combination which comprises (a) conduit means adapted to be positioned in a borehole to provide a path for the circulation of drilling fluid from the surface to the bottom of said borehole;

(b) piston means supported at the lower end portion of said conduit means above and adjacent to the bottom of said borehole;

(c) a fluid passageway extending through said piston means and in fluid communication with said conduit means and said bottom of said borehole; and

(d) means for reciprocating said piston means at a frequency and magnitude such that when said piston is spaced from the bottom of said borehole acoustic pressure pulses are generated in the drilling fluid to effect drilling of the borehole, said means for reciprocating said piston means being supported by the lower end portion of said conduit means and in turn supporting thereon said piston means.

2. The apparatus of claim 1 wherein said means for reciprocating said piston means comprises a fluid turbine type motor.

3. The apparatus of claim 1 wherein said means for reciprocating said piston means comprises a magnetostrictive device.

4. The apparatus of claim 1 wherein said means for reciprocating said piston means include a jet-edge generator including a vibrating bar having a pair of knife edges at opposite ends thereof, means to direct drilling fluid against said knife edges, and means uniting said piston to said vibrating bar substantially at the middle thereof.

5. The apparatus of claim 1, further comprising means to regulate said means for reciprocating said piston means in correlation with the depth of the borehole to maintain the generation of said acoustic pressure pulses for effecting the drilling of the borehole.

6. The apparatus of claim 1, further comprising means to regulate said means for reciprocating said piston means to obtain cavitation in the drilling fluid beneath said piston means.

7. In apparatus for drilling a borehole in an earth formation, the combination which comprises (a) substantially rigid conduit means adapted to be positioned in a borehole;

(b) piston means supported at the lower end portion of said conduit means above and adjacent to the bottom of said borehole;

(c) means for reciprocating said piston means at a frequency and magnitude such that when said piston is spaced from the bottom of said borehole acoustic pressure pulses are generated in the drilling fluid to effect drilling of the borehole with the concomitant formation of rock cuttings at the bottom of the borehole, said means being supported by the lower end portion of said conduit means and in turn supporting thereon said piston means; and

(d) means including said conduit means to provide a path for the circulation of drilling fluid from the surface down the borehole into contact with said cuttings and thereafter up the borehole to remove said cuttings.

8. In apparatus for drilling a borehole in an earth formation, the combination which comprises (a) substantially rigid conduit means adapted to be positioned in a borehole;

(b) piston means supported at the lower end portion of said conduit means above and adjacent to the bottom of said borehole;

(0) means for maintaining said piston means spaced from about 2 millimeters to about 2 borehole diameters from the bottom of said borehole;

(d) means for reciprocating said piston means at a frequency and magnitude such that when said piston is spaced from the bottom of said borehole acoustic pressure pulses are generated in the drilling fluid to effect drilling of the borehole with the concomitant formation of rock cuttings at the bottom of the borehole, said means being supported by the lower end portion of said conduit means and in turn supporting thereon said piston means; and

(e) means including said conduit means to provide a path for the circulation of drilling fluid from the surface down the borehole into contact with said cuttings and thereafter up the borehole to remove said cuttings.

9. In a method of drilling a downwardly directed borehole in an earth formation, the steps which comprise (a) positioning a piston spaced from but closely adjacent to the bottom of said borehole;

(b) filling the space between the piston and the bottom of the borehole with fluid;

(c) reciprocating said piston at a stroke and frequency such that acoustic pressure pulses are generated in the fluid filled space of a magnitude sufficient to effect failure of the formation forming rock cuttings at the bottom of the borehole; and

(d) circulating drilling fluid through the borehole to elfect removal of the rock cuttings.

10. In a method according to claim 9, further comprising regulating the reciprocation of said piston so that its velocity is suflicient to cause cavitation in accordance with the following formula:

where V is the maximum velocity of the piston, P is the static pressure within the fluid, g is the acceleration produ-ced by gravity, p is the density of the fluid, and

C is the speed of sound in the fluid.

11. In a method of drilling a downwardly directed borehole in an earth formation, the steps which comprise (a) positioning means for generating acoustic pressure pulses spaced from but closely adjacent to the bottom of the borehole;

(b) filling the space between said means for generating acoustic pressure pulses and the bottom of the borehole with fluid to provide a fluid filled zone at the bottom of the borehole;

(c) circulating drilling fluid through the borehole at least a portion of which fluid circulates through the fluid filled zone;

(d) actuating said means for generating acoustic pressure pulses to generate acoustic pressure pulses in the fluid in said zone under conditions such that drilling is obtained; and

(e) moving said means for generating acoustic pressure pulses downwardly at a rate commensurate with the rate of drilling of said borehole by said drilling fluid to maintain said means for generating acoustic pressure pulses at a substantially uniform distance from the bottom of said borehole.

12. In a method of drilling a downwardly directed borehole in an earth formation, the steps which comprise (a) positioning a piston between about 2 millimeters and about 2 borehole diameters from the bottom of said borehole;

(b) filling the space between the piston and the bottom of the borehole with fluid;

(c) reciprocating said piston at a stroke and frequency such that acoustic pressure pulses are generated in the fluid filled space of a magnitude sufficient to effect failure of the formation forming rock cuttings at the bottom of the borehole; and

(d) circulating drilling fluid through the borehole t0 elfect removal of the rock cuttings.

13. In a method of drilling a downwardly directed borehole in an earth formation, the steps which comprise (a) positioning means for generating acoustic pressure pulses between about 2 millimeters and about 2 borehole diameters from the bottom of the borehole;

(b) filling the space between said means for generating acoustic pressure pulses and the bottom of the borehole with fluid to provide a fluid filled zone at the bottom of the borehole;

(c) circulating drilling fluid through the borehole at least a portion of which fluid circulates through the liquid filled zone;

(d) actuating said means for generating acoustic pressure pulses to generate acoustic pressure pulses in the fluid in said zone under conditions such that drilling is obtained; and

(e) moving said means for generating acoustic pressure pulses downwardly at a rate commensurate with the rate of drilling of said borehole by said drilling fluid to maintain said means for generating acoustic pressure pulses at between about 2 millimeters and about 2 borehole diameters from the bottom of said borehole.

14. In a method according to claim 13, further comprising regulating the actuation of said means for generating acoustic pressure pulses in order to produce cavitation in the fluid filled zone.

References Cited by the Examiner UNITED STATES PATENTS 2,796,129 6/ 1957 Brandon 166-9 2,816,612 12/1957 Hut-chison et a1. 166-177 2,831,668 4/1958 Skowronski -56 X 2,866,509 12/1958 Brandon 166-177 2,871,943 2/1959 Bodine 166-177 X 2,951,682 9/1960 Bocher 175-56 2,989,130 6/1961 Mathewson et al 175-56 3,016,095 1/1962 Bodine 166-177 3,045,749 7/1962 Brandon 166-177 X 3,094,176 6/1963 Cook 175-56 X CHARLES E. OCONNELL, Primary Examiner.

R. E. FAVREAU, Assistant Examiner. 

1. IN APPARATUS FOR DRILLING A BOREHOLE IN AN EARTH FORMATION, THE COMBINATION WHICH COMPRISES (A) CONDUIT MEANS ADAPTED TO BE POSITIONED IN A BOREHOLE TO PROVIDE A PATH FOR THE CIRCULATION OF DRILLING FLUID FROM THE SURFACE TO THE BOTTOM OF SAID BOREHOLE; (B) PISTON MEANS SUPPORTED AT THE LOWER END PORTION OF SAID CONDUIT MEANS ABOVE AND ADJACENT TO THE BOTTOM OF SAID BOREHOLE; (C) A FLUID PASSAGEWAY EXTENDING THROUGH SAID PISTON MEANS AND IN FLUID COMMUNICATION WITH SAID CONDUIT MEANS AND SAID BOTTOM OF SAID BOREHOLE; AND (D) MEANS FOR RECIPROCATING SAID PISTON MEANS AT A FREQUENCY AND MAGNITUDE SUCH THAT WHEN SAID PISTON IS SPACED FROM THE BOTTOM OF SAID BOREHOLE ACOUSTIC PRESSURE PULSES ARE GENERATED IN THE DRILLING FLUID TO EFFECT DRILLING OF THE BOREHOLE, SAID MEANS FOR RECIPROCATING SAID PISTON MEANS BEING SUPPORTED BY THE LOWER END PORTION OF SAID CONDUIT MEANS AND IN TURN SUPPORTING THEREON SAID PISTON MEANS. 